(a) Field of the Invention
The present invention relates to disc prostheses for the rachis and, more particularly, to disc prostheses installable through anterior surgery.
(b) Description of Prior Art
When a cervical disc is anteriorly removed (e.g. using the Smith-Robinson surgical technique) from between two adjacent vertebrae, for instance to liberate roots and/or medulla which are compressed by a degenerated disc or to remove a damaged disc, it is known to fuse both these vertebrae together (i.e. osteosynthesis by way of an anterior cervical plate) to provide stability to the rachis although this results in loss of mobility and damping. This rigidification (on one or more successive discs) induces greater stresses to the natural discs located adjacently above and below the removed disc(s) thereby causing a premature ageing of these natural discs, and also creates experimental conditions for the formation of discal hernias on the adjacent levels.
U.S. Pat. No. 5,258,031 issued to Salib et al. discloses a non-compressible prosthetic disc of the ball-and-socket type where the male and female members are fixed to respective upper and lower plates that are secured to adjacent upper and lower vertebrae by way of screws. The disc prosthesis is thus adapted to replace a natural disc of the lumbar spine and it provides six degrees of freedom such as to substantially reproduce the normal intervertebral pivoting movements, except compression.
U.S. Pat. Nos. 5,865,846 and 6,001,130 both issued to Bryan et al. respectively on Feb. 2 and Dec. 14, 1999 are similar (although the latter is more detailed) in each teaching a disc prosthesis comprising a resilient body, of varying stiffness from a substantially stiff exterior annular gasket to a relatively supple central nucleus. The disc prosthesis is adapted to be installed in the intervertebral space with concave-convex elements at least partly surrounding the resilient body to retain it in the intervertebral space. These elements include a pair of L-shaped supports mounted to respective adjacent upper and lower vertebrae with screws that extend through the vertical leg or wing sections of the L-shaped supports. The horizontal leg or wing sections of the L-shaped supports extend in the intervertebral space and surround the resilient body on opposite upper and lower sides thereof. It is possible for the vertical and horizontal leg sections of each L-shaped support to be hinged together at the anterior faces of the vertebrae but only for adjustment during installation of the prosthesis and not to act as a hinge after installation. There may be two or more disc prostheses disposed in series between three or more adjacent vertebrae.
U.S. Pat. No. 5,755,796 issued on May 26, 1998 to Ibo et al. discloses a cervical intervertebral disc prosthesis also of the ball-and-socket type which allows for a pivotal movement between two adjacent vertebrae.
U.S. Pat. No. 5,556,431 issued on Sep. 10, 1996 to Büttner-Janz also teaches a disc prosthesis somewhat similar in function to that of aforementioned U.S. Pat. No. 6,001,130, although its two anchoring plates do not extend along the anterior faces of the vertebrae. The screws holding the anchoring plates to the vertebrae are engaged in the vertebrae from the intervertebral face thereof. The prosthesis core has a peripheral rim to limit its range of movements. Anchoring teeth are provided on the plates for penetrating, under load, the vertebrae.
U.S. Pat. No. 3,426,364 issued on Feb. 11, 1969 to Lumb describes a spinal prosthesis to replace natural vertebrae which had to be removed. A spring member may extend in the prosthesis.
U.S. Pat. No. 5,562,738 issued to Boyd et al. on Oct. 8, 1996 is similar to above-described U.S. Pat. No. 5,258,031.
U.S. Pat. No. 5,171,280 issued on Dec. 15, 1992 to Baumgartner discloses an inter-vertebral prosthesis which includes a coiler body able to rotate onto a fixed base with a flexible elastic hollow body extending from the coiler body and adapted to receive therein a filling medium through a valve. The prosthesis, once implanted and filled with an incompressible medium, is able to absorb radial forces exerted upon the periphery via the incompressible medium in the elastic hollow body. The prosthesis can be inserted in the intervertebral region through a small opening.
U.S. Pat. No. 3,875,595 issued on Apr. 8, 1975 to Froning discloses an inter-vertebral disc prosthesis in the form of a collapsible plastic bladder-like member that has the shape of the nucleus pulposis of a natural inter-vertebral disc. After removal of the degenerated natural nucleus pulposis, the prosthesis, in its collapsed position, is inserted through a stem and into the inter-somatic space, and a filling medium is then inserted through the stem and into the prosthesis to inflate it to a natural form. The stem is then severed just upstream of a valve thereof such that the valve remains implanted with the prosthesis.
U.S. Pat. No. 4,772,287 and U.S Pat. No. 4,904,260 which issued respectively on Sep. 20, 1998 and Feb. 27, 1990 both in the names of Ray et al. describe the implantation of two prosthetic disc capsules side-by-side into a damaged disc of a human spine.
U.S. Pat. No. 6,022,376 issued on Feb. 8, 2000 to Assell et al. discloses a capsule-shaped prosthetic spinal disc nucleus for implantation into a human intradiscal space, made of a substantially inelastic constraining jacket surrounding an amorphous polymer core with the constraining jacket having a fixed maximum volume and defining a height, while the amorphous polymer core fills an initial volume of the constraining jacket and develops an internal pressure.
U.S. Pat. No. 5,192,326 and U.S. Pat. No. 5,047,055 which issued respectively on Mar. 9, 1993 and Sep. 10, 1991 both in the name of Bao et al. teach a prosthetic nucleus adapted to be implanted in the intersomatic space of a spine and which is formed of a multiplicity of hydrogel beads which are covered by a semi-permeable membrane. This prosthetic nucleus is adapted to conform, when hydrated, to the general shape of the natural nucleus. The prosthetic nucleus is surrounded by the natural annulus fibrous. Vertebral end plates cover the superior and inferior faces of the prosthetic nucleus.
U.S. Pat. No. 4,863,477 issued on Sep. 5, 1989 to Monson discloses a synthetic inter-vertebral disc prosthesis which is made of two halves which, after having been joined together, are implanted in the intersomatic space in place of a removed natural disc. A fluid, such as a saline solution, is then injected into the interior cavity of the prosthesis to provide the required amount of resiliency in the disc prosthesis thereby restoring proper vertebral spacing and facilitating flexibility of the spine.
U.S. Pat. No. 5,976,186 issued on Nov. 2, 1999 to Bao et al. discloses a hydrogel inter-vertebral disc nucleus adapted to be inserted in the inter-somatic space through an opening in the natural annulus for replacing the natural nucleus. The hydrogel disc is adapted to essentially fill the inter-vertebral nuclear disc cavity upon absorbing sufficient water from the body fluids.